The present invention is concerned with biomass pyrolysis, also referred to as thermolysis. Particularly, the present invention is concerned with an apparatus for converting biomass to pyrolysis bio-oil with high yield and minimal byproducts.
Solid biomass, typically wood, energy crops, and agricultural residues, can be converted to useful products, such as fuels or chemicals, by the application of heat. The most common example of thermal conversion is combustion, where air is added and the entire biomass feed material is burned to provide hot combustion gases for the production of heat, steam, and/or work. A second example is gasification, where the biomass feedstock is reacted at high temperature (typically >700° C.) without combustion with a controlled amount of oxygen and/or steam to produce a combustible fuel gas. The combustible gas, known as producer gas or synthesis gas (syngas), is comprised of carbon monoxide, hydrogen, and carbon dioxide. A final example of thermal conversion is pyrolysis where the solid biomass is converted to liquid and char, along with a gaseous byproduct, through reaction at high temperatures (typically 350-550° C.) in a substantially oxygen-free environment.
In a generic sense, pyrolysis is the conversion of biomass to a liquid and/or char by the action of heat in a substantially non-oxidizing environment. The char is a carbonaceous solid (including traces of inorganic ash) that remains after the majority of the organic material has been removed from the biomass through pyrolysis. A small quantity of combustible gas is also a typical byproduct. Historically, pyrolysis was a relatively slow process where the resulting liquid product was a viscous tar. Conventional slow pyrolysis has typically taken place at temperatures below 400° C. and at reactor residence times ranging from several seconds to minutes. The residence time can be measured in hours for some slow pyrolysis processes used for charcoal production.
A more modern form of pyrolysis, termed fast (or flash) pyrolysis, was discovered in the late 1970s when researchers noted that a high yield of a light pourable liquid was possible from biomass. In fact, liquid yields approaching 75% of the weight of the input woody biomass material were possible if the pyrolysis temperatures were moderately raised (compared to slow pyrolysis) and the conversion was allowed to take place over a very short time period, typically less than 5 seconds.
The homogeneous liquid product from fast pyrolysis, which is generally opaque dark brown in color, has since become known as bio-oil (or pyrolysis oil). Bio-oil is mixture of various oxygenated hydrocarbons and water derived from depolymerization of the lignocellulosic biomass material. Bio-oil is suitable as a fuel for combustion in boilers and for use in modified diesel engines and stationary turbines. This liquid product is in stark contrast to slow pyrolysis, which produces a thick, low quality, two-phase tar-aqueous mixture in very low yields.
In practice, the fast pyrolysis of solid biomass causes the major part of its solid organic material to be rapidly transformed into a vapor phase. This vapor phase contains both non-condensable gases (including methane, hydrogen, carbon monoxide, carbon dioxide and other light hydrocarbons) and condensable organic vapors. The condensable vapors are cooled and collected as the final liquid bio-oil product. The yield and value of this bio-oil product is a strong function of the method and efficiency of the downstream capture and recovery system. The condensable vapors produced during fast pyrolysis continue to react in the vapor phase, and therefore must be quickly cooled or “quenched” in the downstream process before they can deteriorate into lower value tarry solids or non-condensable gaseous products. Of particular concern is the secondary cracking of the product vapors into smaller molecules that cannot be condensed and incorporated into the desired liquid bio-oil product.
There are two main types of biomass pyrolysis reactors in service today as commercial-scale or pilot-scale plants—bubbling fluidized beds and circulated (or transported) fluidized beds. In both types of reactors, a fluidized bed is used to transfer heat quickly to the biomass feedstock particles. The fluidized bed is a solid/gas mixture that behaves as a fluid due to the introduction of gas flow through the particulate bed medium. A refractory, inorganic material such as sand is often used for this the bed medium. Optionally, the bed medium can contain a catalyst that promotes the formation of more desirable product compounds. The biomass is fed into the reactor where it contacts the bed and the heat is conducted into the biomass particle which begins to pyrolyze, and the organic vapors then are transported out of the biomass particle. The bed is supported underneath by a porous flow distributor that ensures even fluidization. As the biomass particles react, they lose mass as vapors are produced. The outside of the particles are also abraded by the bed medium, exposing fresh biomass material, and leading to progressively smaller particles. In the bubbling fluidized bed reactor, the fluidization velocity is low enough that the bed medium is not carried out of the top of the reactor by the upward flow of the fluidizing gas. The vapors and small char particles are elutriated from the bed and carried out of the top of the reactor. No additional separator (disengager) is required separate the bed medium from the products. In contrast, in a circulating fluidized bed reactor, the vapors, char, and bed medium are all carried out of the top of the reactor. Typically the char and bed medium are then separated from the vapors using a separator (or disengager) such as a cyclone separator or the like. The bed medium is then reheated, often by combustion of the char and excess process gas, and returned to the reactor. The bubbling fluidized bed reactor has an advantage over the circulating fluidized bed in that the bed material is separated from the pyrolysis vapors at an earlier stage in the process, inside the reactor vessel rather than later in a separator after leaving the reactor. This early separation provides the opportunity to quench the pyrolysis vapors more quickly and effectively. Another advantage of the bubbling fluidized bed reactor is that the lower portion of the reactor is dense with bed medium (e.g. sand) rather than having the bed medium dispersed throughout the whole reactor. The dense bed provides better heat transfer to the biomass and abrasion of the biomass and thereby improves the reaction rate and enables the short vapor residence time characteristic of fast pyrolysis.
Boroson et al. (AIChE Journal, Vol. 35, No. 1, pp. 120-128, 1989) measured the rate of secondary cracking reactions for primary wood bio-oil and found the bio-oil yield decreased by roughly 5% after only 2 s of exposure at 500° C. Other researchers have measured similar rates, although generally they have been faster indicating the potential for even greater loss of yield with inadequate quenching. Regardless, it has been clearly demonstrated that limiting the exposure of the pyrolysis vapors to high temperatures is crucial to maintaining a high liquid yield. That said, until the char particles have been removed, the temperature must be maintained above the dew point of the bio-oil (i.e., the temperature that corresponds to the condensation of the first drop of liquid). Otherwise, the liquid product will foul the internal surfaces of the reactor, piping and cyclone separator. In prior biomass pyrolysis reactors (both bubbling fluidized beds and circulating fluidized beds), the quenching was done external to the reactor and downstream of the cyclone. In these reactors, the quenching has been done either indirectly, through a heat exchanger, or directly by spraying the hot vapors with bio-oil that was previously condensed and cooled. As used herein, the term “indirect”, as applied to heat exchange methods, implies that the medium to which heat is principally transferred does not contact the higher temperature material, heat transfer being accomplished via conduction through an intermediate medium such as a tube wall or other barrier. Similar quenching arrangements external to the reactor have been used for condensing the reactive pyrolysis vapors produced during the steam cracking (pyrolysis) of light alkanes to make ethylene in the chemical industry.
U.S. Pat. No. 8,057,641 describes quenching inside the reactor in a circulating fluidized bed reactor for biomass pyrolysis. The quench, as described, necessarily cools the pyrolysis vapors and also the bed medium (i.e., heat carrier), which is later removed in a separator (disengager) downstream of the reactor. Cooling the bed medium is undesirable because it must then be reheated, requiring more heat than otherwise necessary and reducing the overall thermal efficiency of the process. There is a need for improved biomass pyrolysis reactors that provide a rapid quench to minimize yield-robbing secondary cracking reactions while maintaining good thermal efficiency of the process.
An object of our invention is to maximize the liquid yield of bio-oil in the fast pyrolysis of biomass or carbonaceous material with an improved apparatus and method for quenching the product vapors to avoid secondary cracking reactions.
Another object of our invention is to recycle process gas to the bubbling fluidized bed reactor as the quench fluid.
Unlike the known variations of fast pyrolysis reactors, the present invention improves on them by introducing the quench fluid inside the reactor in a bubbling fluidized bed reactor. The quench fluid is introduced above and downstream of the bed thereby only cooling the vapors and not the bed medium. In addition to reducing the cooling requirements for quenching, the rate of quenching is increased because of the lower thermal capacity of the quenched stream. This reactor configuration avoids the energy-wasting task of reheating the bed medium. The bed medium is much denser than the vapors, therefore cooling only the vapors dramatically reduces the thermal capacity of the stream being cooled, allowing the quenching to occur significantly faster. The temperature is rapidly decreased by the introduction of the quench fluid by rapid and efficient direct heat transfer. The temperature is maintained above the dew point of the bio-oil but below the temperature where significant secondary cracking occurs, until the vapors leave the reactor and pass through the char separator that removes the char from the product vapor. The vapor is then further cooled in a second stage where the bio-oil is condensed and collected. Maintaining the temperature above the dew point of the bio-oil upstream of the condenser eliminates fouling of the internal surfaces of the reactor, char separator, and piping due to the condensation of the heavy fraction of the bio-oil. Char is known to catalyze the secondary cracking of bio-oil to non-condensable gases. Therefore the char is separated from the reactor as soon as possible to minimize these reactions.